1. Technical Field
An aspect of the present invention relates to a sheet feeding device having a shifting unit to move a holder plate, holding a stack of sheets, toward a sheet feeding roller and an image forming apparatus having the sheet feeding device.
2. Related Art
Conventionally, an image forming apparatus having a detachable sheet cassette, in which a stack of recording sheets are contained, in a bottom portion thereof has been known. The stack of recording sheets are held on a swingable holder plate, which is installed on a bottom of the sheet cassette, so that the recording sheets can be picked up by a feed roller and fed in a sheet feeding path to a printing unit. The picked-up recording sheets are separated in between a separator roller and a separator pad so that solely a topmost sheet can be fed in the sheet feeding path at a time. Such a sheet feeding device is disclosed in, for example, Japanese Patent Provisional Publication No. 2006-96479.
According to the disclosure, the holder plate is provided in the sheet cassette with a side further from the feed roller being rotatably attached to a bottom of the sheet cassette so that a side of the holder plate nearer to the feed roller can be shifted. Thus, the sheet stack is maintained in an uplifted position in the sheet cassette and a topmost sheet in the sheet stack reaches the feed roller to be fed even when a level of the sheet stack in the sheet cassette is low.
However, when the holder plate is pivoted about the further side and the nearer side is uplifted, an angle of the topmost sheet with respect to the feed roller varies from moderate to acute as the level of the sheet stack becomes lower. In this regard, because the angle of the topmost sheet with respect to the feed roller is not constant, the sheets may not always be fed in the sheet feeding path properly. Namely, when a plane including the topmost sheet is not substantially inclined with respect to the feed roller, the recording sheet fed by the feed roller may reach solely to the separator pad but not reach in between the separator roller and the separator pad (see FIG. 10A). Thus, idle feeding, in which the sheet is not forwarded further, may occur. Meanwhile, when a plane including the topmost sheet is inclined at an excessively acute angle, the recording sheet may be nipped with the separator roller and an upper portion of the separator pad (see FIG. 10B). In this event, double-feeding, in which two or more sheets picked up at one time are forwarded further, may occur.
In order to prevent the angle variation, a structure to uplift the holder plate with a topmost plane of the sheet stack being maintained to be parallel with the bottom of the cassette has been suggested in, for example, Japanese Patent Provisional Publication No. 2002-87622. According to the configuration, when the picked-up sheet is fed in the sheet feeding path to a printing unit, which is arranged in an upper portion of the image forming apparatus above the cassette, the sheet is turned acutely along the sheet feeding path curved at nearly 180 degrees, and therefore, a size of the entire image forming apparatus tends to be larger. In order to maintain the apparatus downsized, it is more advantageous that the holder plate is inclined even at a small angle to maintain the side of the holder plate nearer to the feed roller higher. Meanwhile, a structure to maintain the holder plate with the sides both nearer to and further from the feed roller inclined and uplift simultaneously is suggested in, for example, Japanese Patent Provisional Publication No. H10-203655. According to the disclosure, the further side of the holder plate is guided along guide grooves, which are formed in the cassette and angled toward the feed roller, to follow the nearer side of the holder plate being uplifted.